Planographic printing plates



United States Patent-O 3,227,975 PLANOGRAPHIC PRINTING PLATES Samuel Guastella, Westminster, Edward J. Guerra, Fitchburg, D. D. Uong, Leominster, and Wyman F. Uhl,

Ashby, Mass., assignors, by mesne assignments, to

Fitchburg Paper Company, a subsidiary of Litton Industries, Inc, a corporation of Delaware No Drawing. Filed Apr. 4, 1961, Ser. No. 100,537

6 Claims. (Cl. nib-149.2)

Our invention relates to planographic printing plates, and particularly to improved paper base planographic plates and a rapid and simplified method of manufacturing the same.

In planographic printing, the print is formed by pressing the surface to be inked against a smooth plate which has been selectively inked in areas defining the reverse of the print. The process requires a plate having imaged areas which are receptive to the ink, with the remaining, unimaged areas being ink repellent so that by contacting the plate with an inking roller, ink is applied only to the imaged areas. Ordinarily the plate is first wet outwith an aqueous solution to render the unimaged areas incapable of picking up the ink, while the imaged areas, having an oleophilic, hydrophobic character, remain dry. Then, by contacting the plate with an inking roller, ink is applied selectively to the imaged areas.

Plates commonly available consist of a suitable backing material, typically metal, plastic or paper, having a surface which may be uniformly wet with water but which also possesses an affinity for the image. A characteristic of these plates is a durable surface capable of being wet with .water and rendered thoroughly ink repellent while also being highly retentive of the imaging material which is repellent to Water.

Paper base planographic plates are commonly used for short and medium run work, and are particularly suitable for ofiice work because they can be imaged directly by marking or typing with readily available pencils, crayons, ball-point pens, or typewriter ribbons. Such plates generally comprise a clay-coated paper base having a surface covering of a hydrophilic, but insoluble, coating material which ordinarily includes a pigment filler such as clay, blanc fixe, colloidal silica or the like. T e surface must possess a rather critical balance between its hydrophilic and oleophilic characteristics so as to be readily wet with water, and also retentive of ink-receptive imaging material, which is generally oleaginous. The success of any plate depends on its surface being durable so as to resist deterioration from repeated wett-ings, inkings, and pressings, while retaining its water receptive and image retentive properties. Paper base plate suitable for direct imaging should also be sufiiciently durable to permit erasures without destruction of the planographic surface.

Paper plates currently avail-able are generally made by coating a coated paper base stock with an aqueous emulsion, solution or suspension containing a film-forming material, such as casein, starch, carboxy methyl cellulose, albumin or polyvinyl alcohol, then drying the coating and further processing the sheet to render the coating insoluble. This type of processing, while producing satisfactory plates, is relatively complex in that successive operations are necessary, and each of them requires careful control of both the compositions and the conditions of application.

Numerous disadvantages also result from the application of the planographic coating from a water base. A tendency to curl may develop which requires still further treatment, such as coating the backside, to reduce the curl or to avoid other complications impairing usefulness. Moreover, the surface of the base stock, which ordinarily carries a clay coating, may become disrupted during the coating operation so as to require calendering, super- US. Patent No. 3,083,639.

aware, the most promising development in the art prior "ice calendering, or other remedial treatment after the coating operation.

We have found that it is possible to make a satisfactory planographic printing plate by treating a non-planographic paper base with an essentially aqueous solution of colloidal silica, and for example, we have had reasonably successful results with plates prepared by treating a paper base With a suspension of Ludox SM colloidal silica, as obtained from E. I. du Pout de Nemours and Co., Inc., in an aqueous solution containing between 30 and 70 volume percent of acetone, the suspension containing 1 percent silica solids by weight.

However, the difliculties encountered in manufacturing paper plates by methods employing aqueous systems may be avoided by making plates by treating a paper base with essentially organic solutions. Among such solutions, we have successfully employed an organic suspension containing 1 percent of Davidson 79 silica gel in equalvolumes of methyl ethyl ketone and methanol, and We have also employed a 1 percent suspension of Cab-O-Sil M5 silica in ethyl ether. Still better results may be obtained by employing dispersions of silica obtained by hydrolizing ethyl silicate, in any of the ways taught in United States application for Letters Patent Serial No. 824,942, filed July 6, 1959, by John Frank Thurlow, for Pl-anographic Printing Plates and Processes for Making the Same, now Thus, so far as we are to our invention has been the plates prepared in accordance with the afores-aid application Serial No. 824,942, employing a planographic coating of hydrolized ethyl silicate.

However, planographic plates made in accordance with the aforesaid application still leave something to be desired in rapidity of image buildup, cleanup characteristics, practical length of run, era-sure property, and ink spotting. Accordingly, it is a primary object of our invention to provide a planographic printing plate improved in all of the characteristics over the best prior planographic plates.

.11: is a further object of our invention to provide an improved treating suspension for imparting planographic properties to non-planographic surfaces.

It is a further object of our invention to provide an improved process for making high quality planographic printing plates which'is rapid, simple and inexpensive.

Briefly described, a lanographi-c printing plate in accordance with our invention comprises a non-planographic flexible base on the surface of which aredispersed-particles of a partially hydrolized alkyl silicate. By microscopic examination, these particles are found to be relatively unagglomcrated and to be of a characteristic range of sizes. Upon chemical analysis, the surface composition of our improved plate is found to have a characteristic ratio of silicon to organic constituents.

Our improved plates are manufactured, in accordance with a preferred embodiment of our invention, by treating a paper base sheet-coated with a non-planographic coating of clay and latex with an improved treating suspension prepared by heating a suspension of a lower alkyl silicate in a'solution including a water-miscible, polar organic solvent'consisting at least in part of dimethyl formamide ordimethyl acct-amide, water, and ammonia or an alkyl amine.

The application of a partially hydrolized alkyl silicate suspension in accordance with our invention will impart planographic properties to any kind of paper base whose surface is 'not too oleophilic or hydrophilic. Best results, however, are obtained when the base sheet is a clay coated paper, in which the clay coating serves as a barrier capable of preventing the penetration of water into the paper base. The application of barrier coats is commonly employed for this purpose and is not novel with plates prepared according to this invention. However, a barrier coat is necessary if a plate of highest durability and performance is to be produced, and may in that sense be considered as contributing to the ultimate utility of the invention.

While many suitable clay coated papers are known to the art which may be successfully employed in the practice of our invention, the following is a formulation that we have found most satisfactory:

CLAY COATING FORMULA Parts by Weight Clay 100 Casein 18.0

Latex 6.0

It is important that this coating, or other coatings which may be applied to a flexible base in the practice of our invention, be free of deformers, stearates, plasticizers and other oily substances that might impart oleophilic properties to our planographic surface.

The latex in the coating may be butadiene-styrene, such as Dow 512 R, available from the Dow Chemical Company. It is provided to make the coated base waterproof and flexible, and to impart wet-rub resistance to the clay coating. It also contributes to image retention.

The formulation of the novel treating suspension of our invention, in accordance with the preferred embodiment thereof, may best be understood from the following examples, which we have used to make planographic plates of excellent quality capable of giving more than 2,000 useful copies:

Example I Parts by volume Condensed ethyl silicate 28.8 Dimethyl formamide 36.9 Methanol a 24.6 Aqueous NH OH (28% NH 4.4 Distilled H O 3.3

The ammonia is diluted with the water. The condensed ethyl silicate, dimethyl formamide, and methanol are combined, and then added to the ammonia solution in a vessel provided with circulating water coils. The vessel is closed, and the contents are reacted at elevated temperatures with a gradual rate of temperature rise from 70 F. to 131 F. over a 6 hour period.

The rise of the temperature curve should be a smooth one with no sudden, erratic maxirna or minirna. The temperature rise is controlled by controlling the temperature and rate of flow of the water in the cooling coils.

The reaction temperature and time affect the colloidal particle size and the degree of substitution of the ethyl groups in the reaction product. The particle size of the planographic coatings will influence the image retention on the plate.

For a desirable predetermined end product, rate of temperature increase correlates with the percentage of water and ammonia contained in the formula. In other words, the formula may be designed for rapid or slow reaction depending on the reaction equipment.

Example II Parts by volume Condensed ethyl silicate 28.8 Dimethyl acetamide 36.9 Methanol 24.6 Aqueous ammonia (28% NH 4.4 Distilled H O 3.3

When reacted as described in connection with Example I above, the formulation of Example 11 produced an excellent treating suspension having the sa-rne properties as the formulation of Example I.

The application of partially hydrolized ethyl silicate from the above suspensions to a suitable base produces a firmly bonded coating which is microscopically grained,

because of the particulate nature of the siliceous reaction product, but which aproaches a uniform film. This coating differs from that obtained from an aqueous dispersion of silica, in that there is a tendency for the silica in the latter to agglomerate, and in that the coat that is formed from the aqueous dispersion is in the nature of relatively few, highly concentrated, deposits of agglomerated particles which adhere with much less strength and are consequently easily removed. The surface formed from an aqueous suspension is neither durable nor possesses good planographic properties, probably because a uniform siliceous deposit is not formed. The coating formed by our suspension differs from those obtained from organic dispersions of silica in that the siliceous coating is much more firmly bonded to the base. Also, the wearing properties are generally superior, particularly with regard to the ability of the planographic surface to withstand erasures. The application of a partially hydrolized ethyl silicate coating from an organic suspension results in a finely textured siliceous surface, with the siliceous particles covering substantially all of the base coating and constituting a textured surface. The siliceous nature of the surface renders it readily wet with water, While its organic constituent, finely textured nature can be visualize-d as providing excellent image retention.

We have analyzed the films deposi ed from our suspensions by drying coatings of the suspensions on a Mylar polyester film, and then subjecting the deposits to chemical analysis. From a suspension prepared in accordance with Example I above, we obtained deposits having the following analysis, in terms of percent by weight.

Constituent: Percent by wt. C 6.71

C H O 3.15

Thus, the optimum degree of partial hydrolysis would appear to be that resulting in film deposits incorporating approximately 6.7 percent of carbon. Since the complete hydrolysis of the silicate would leave essentially no organic constituents in the film deposits, the percentage of carbon is an eifective indication of the degree of hydrolysis. We have found that the extent of hydrolysis determines the relative oleophilicity and hydrophilicity of the planographic surface formed, and that the partial hydrolysis may be carried out far enough to leave only about 2 percent of carbon in the film deposits, or may be restricted so long as the dried film retains a sufficiently hydrophilic character. That is, the hydrolysis must be carried at least to the extent that paper plates made from the resulting suspensions and wet with water will not pick up ink in their unimaged areas, as can readily be determined by routine experimentation without the exercise of invention.

It should be understood that some variation in the formulation of the treating suspension of our invention can be made. For example, we have experimented with suspensions in which the ammonia was varied from 25% less than that given above to 25 more, by volume, with reasonably successful results at both extremities. Examination of electron microscope photomicrographs of surface deposits from such suspensions containing excess, deficient, and normal amounts of ammonia (as defined by the above example) show a progressive variation in particle size, with the smaller particles occurring in the deposit from the ammonia-deficient suspension, and the larger particles occurring in the deposits from the excessively ammoniated suspensions. The particle size for optimum results appears, from measurement made on these photomicrographs, to lie in the range from about 10 to millimicrons. As another indication of the particle size, the suspended particles are sufliciently colloidal to exhibit a Tyndal effect, without being sufficiently small to form a clear solution nor sufficiently large to give a milky suspension.

We have also made planographic plates using diethyl amine as the hydrolysis agent. These plates, while not meeting the standards of plates prepared with ammonia as the hydrolysis agent, are still of good quality. Thus, it is apparent that in the broader aspects of our invention diethyl amine, or, from known chemical principles, any of the lower alkyl amines, can be employed as the hydrolysis agent. On the other hand, other bases, such as tie carbonates, bicarbonates and hydroxides of sodium and potassium, cannot be successfully employed.

While the amount of water used in Examples I and II appears to be the optimum, we have found that the Water content may be varied over a considerable range. Thus, we have prepared planographic plates from suspensions to which no water other than that present in the aqueous ammonia has been added, and also from suspensions containing ten times the amount of water added in Examples I and II. As water is increased or decreased in the formulation of the treating suspension, from the optimum quantity, a gradual decrease in the quality of the plates is observed, although all have useful planographic properties.

The organic solvent used in preparing the treating suspensions of our invention should contain at least a portion of either dimethyl acetamide or dimethyl formamide, since we have found no other solvent that will produce plates of the same quality. The balance of the solvent is preferably methanol, but methyl ethyl ketone, and presumably any other unreactive, water-miscible, polar organic solvent, may be employed. From experiments in which the proportions of dimethyl formamide and methanol were varied, from 25 percent by volume of dimethyl formamide to pure dimethyl formarnide, we have found that optimum results are secured with the organic solvent of Example 1, containing 60 percent dimethyl formamide and 40 percent methanol by volume, but that planographic plates can be made with all of the variations. In decreasing order, the acceptibility of plates made with the various solvent proportions is as follows; values are in parts by volume.

Dimethyl Formamide: Methanol 60 40 Although the examples given employ only condensed ethyl silicate as the siliceous starting material, it is believed apparent from known principles of chemistry that other alkyl silicates, and particularly the lower alkyl silicates, would have partial hydrolysis, products of similar properties. Accordingly, other alkyl silicates may be employed if so desired without departing from the scope of our invention.

While we have given various examples and illustrations of our invention, many changes and variations will be apparent to those skilled in the art upon reading our description, and these can be made without departing from the scope of our invention.

Having thus described our invention, what we claim is:

l. A planographic printing plate comprising a paper base sheet having a printing face carrying as its principal planographic component particles of colloidal dimensions of between 10 millimicrons and 50 millimicrons of the product obtained by partially hydrolyzing a lower alkyl silicate by reacting water and ammonia with a mixture of said lower alkyl silicate and a solvent which solvent comprises 25 to percent of material selected from the class consisting of dimethyl formamide and dimethyl acetamide and the balance of which solvent consists of an unreactive water-miscible polar organic solvent, the rela tive proportions of reactants being:

Parts by volume Alkyl silicate 28.8 Ammonia as aqueous 28% 3.3-5.5 Additional water 0-33 thereby to produce a siliceous hydrolyzate with residual organic constituents containing at least about 2% of the weight of said hydrolyzate of carbon, but less than enough to impair the essential water receptive surface characteristics of the planographic surface.

2. The product defined by claim 1, wherein the alkyl silicate is ethyl silicate.

3. The product defined by claim 1, wherein the alkyl silicate is condensed ethyl silicate the polar organic solvent is methanol, and the proportions of ammonia and water are respectively about 4.4 and 3.3.

4. The method of making paper base planographic printing plates, comprising the steps of partially hydrolyz ing a lower alkyl silicate dispersed in a solvent which comprises from 25 to 100 percent of material selected from the class consisting of dimethyl formamide and dimethyl acetamide and the balance consists essentially of an unreactive water-miscible polar organic solvent, by reacting said silicate with ammonia and water, the relative proportions of reactant being:

Parts by volume Lower alkyl silicate 28.8 Ammonia, as aqueous 28% 3.3-5.5 Additional water 0-33 thereby to produce a reaction product mixture containing a partial hydrolyzate of the lower alkyl silicate in the form of colloidial particles of between 10 millimicrons and 50 millimicrons of siliceous material with residual organic constituents containing at least about 2% of the weight of said siliceous material of carbon, coating the reaction product mixture on a paper base, and drying the coated base.

5. The method defined by claim 4, wherein the alkyl silicate is ethyl silicate.

6. The method defined by claim 4, wherein the alkyl silicate is condensed ethyl silicate the polar organic solvent is methanol, and the proportions of ammonia and water are respectively about 4.4 and 3.3.

References Cited by the Examiner UNITED STATES PATENTS 2,132,443 10/1938 Simons 101-149.2 2,395,880 3/1946 Kirk 260-4488 2,408,654 11/ 1946 Kirk 252-309 3,017,826 1/1962 Salzberg 101-149,2 3,028,804 4/1963 Neugebauer et al. 101-1492 3,083,639 4/1963 Thurlow 101-1492 DAVID KLEIN, Primary'Examiner.

RICHARD D. NEVIUS, Examiner. 

1. A PLANOGRAPHIC PRINTING PLATE COMPRISING A PAPER BASE SHEET HAVING A PRINTING FACE CARRYING AS ITS PRINCIPAL PLANOGRAPHIC COMPONENT PARTICLES OF COOLOIDAL DIMENSIONS OF BETWEEN 10 MILLIMICRONS AND 50 MILLIIMICRONS OF THE PRODUCT OBTAINED BY PARTIALLY HYDROLYZING A LOWER ALKYL SILICATE BY REACTING WATER AND AMMONIA WITH A MIXTURE OF SAID LOWER ALKYL SILICATE AND A SOLVENT WHICH SOLVENT COMPRISES 25 TO 100 PERCENT OF MATERIAL SELECTED FROM THE CLASS CONSISTING OF DIMETHYL FORMAMIDE AND DIMETHYL ACETAMIDE AND THE BALANCE OF WHICH SOLVENT CONSISTS OF AN UNREACTIVE WATER-MISCIBLE POLAR ORGANIC SOLVENT, THE RELATIVE PROPORTIONS OF REACTANTS BEING: 